Long term trends in prevalence of neural tube defects in Europe: population based study

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open access

For numbered affiliations see end of article.

Correspondence to:

B Khoshnood

babak.khoshnood@inserm.fr Cite this as: BMJ 2015;351:h5949 doi: 10.1136/bmj.h5949 Accepted: 19 October 2015

Long term trends in prevalence of neural tube defects in Europe:

population based study

Babak Khoshnood,

¹

Maria Loane,

²

Hermien de Walle,

³

Larraitz Arriola,

⁴

Marie-Claude Addor,

⁵

Ingeborg Barisic,

⁶

Judit Beres,

⁷

Fabrizio Bianchi,

⁸

Carlos Dias,

⁹

Elizabeth Draper,

¹⁰

Ester Garne,

¹¹

Miriam Gatt,

¹²

Martin Haeusler,

¹³

Kari Klungsoyr,

¹⁴

Anna Latos-Bielenska,

¹⁵

Catherine Lynch,

¹⁶

Bob McDonnell,

¹⁷

Vera Nelen,

¹⁸

Amanda J Neville,

¹⁹

Mary T O’Mahony,

²⁰

Annette Queisser-Luft,

²¹

Judith Rankin,

²²

Anke Rissmann,

²³

Annukka Ritvanen,

²⁴

Catherine Rounding,

²⁵

Antonin Sipek,

²⁶

David Tucker,

²⁷

Christine Verellen-Dumoulin,

²⁸

Diana Wellesley,

²⁹

Helen Dolk

²

ABSTRACT Study queStion

What are the long term trends in the total (live births, fetal deaths, and terminations of pregnancy for fetal anomaly) and live birth prevalence of neural tube defects (NTD) in Europe, where many countries have issued recommendations for folic acid

supplementation but a policy for mandatory folic acid fortification of food does not exist?

MethodS

This was a population based, observational study using data on 11 353 cases of NTD not associated with chromosomal anomalies, including 4162 cases of anencephaly and 5776 cases of spina bifida from 28 EUROCAT (European Surveillance of Congenital Anomalies) registries covering approximately 12.5 million births in 19 countries between 1991 and 2011.

The main outcome measures were total and live birth prevalence of NTD, as well as anencephaly and spina bifida, with time trends analysed using random effects Poisson regression models to account for

heterogeneities across registries and splines to model non-linear time trends.

SuMMary anSwer and liMitationS

Overall, the pooled total prevalence of NTD during the study period was 9.1 per 10 000 births. Prevalence of NTD fluctuated slightly but without an obvious

downward trend, with the final estimate of the pooled total prevalence of NTD in 2011 similar to that in 1991.

Estimates from Poisson models that took registry heterogeneities into account showed an annual increase of 4% (prevalence ratio 1.04, 95% confidence interval 1.01 to 1.07) in 1995-99 and a decrease of 3%

per year in 1999-2003 (0.97, 0.95 to 0.99), with stable rates thereafter. The trend patterns for anencephaly and spina bifida were similar, but neither anomaly decreased substantially over time. The live birth prevalence of NTD generally decreased, especially for anencephaly. Registration problems or other data artefacts cannot be excluded as a partial explanation of the observed trends (or lack thereof) in the prevalence of NTD.

what thiS Study addS

In the absence of mandatory fortification, the

prevalence of NTD has not decreased in Europe despite longstanding recommendations aimed at promoting peri-conceptional folic acid supplementation and existence of voluntary folic acid fortification.

Funding, CoMpeting intereStS, data Sharing The study was funded by the European Public Health Commission, EUROCAT Joint Action 2011-2013. HD and ML received support from the European Commission DG Sanco during the conduct of this study. No additional data available.

Introduction

Neural tube defects (NTD) are a major group of severe congenital anomalies that are associated with substantial mortality, morbidity, and long term disability, as well as emotional, psychological, and economic costs.¹² Each year, approximately 5000 fetuses in Europe are affected with NTD. Most of these cases are diagnosed prenatally, and termination of pregnancy for fetal anomaly is by far the most common outcome for fetuses with NTD.^3-5

Convincing evidence shows that peri-conceptional folic acid supplementation can substantially decrease the prevalence of NTD.^6-8 Whereas many countries in Europe have issued recommendations for folic acid supplementation for women of reproductive age, or specifically for those who intend to become pregnant, mandatory fortification programmes do not yet exist in Europe.⁹ A previous study found that in the past these recommendations had not had an appreciable effect on the prevalence of NTD in European countries.¹⁰ A more WhAT IS AlReAdy knoWn on ThIS TopIC

Peri-conceptional supplementation with folic acid can greatly reduce the risk of neural tube defects (NTD)

Although various recommendations for folic acid supplementation have been issued in Europe and elsewhere, important barriers exist for effective implementation of these recommendations

In contrast, mandatory fortification of food staples with folic acid has proved very effective in decreasing the prevalence of NTD

Up to date, population based data on the long term trends of the prevalence of NTD in Europe, where policies for mandatory fortification do not exist, are not available and could help to inform future policies

WhAT ThIS STudy AddS

The prevalence of NTD has not decreased in Europe despite longstanding recommendations aimed at promoting peri-conceptional folic acid supplementation and the existence of voluntary folic acid fortification Policies for mandatory fortification of food staples with folic acid should be

considered as an important and more effective means for prevention of NTD in Europe

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recent but preliminary analysis of the pooled data on the prevalence of NTD in the European Surveillance of Congenital Anomalies (EUROCAT) network suggested that their overall prevalence may have slightly decreased in the period 2004-08.⁵

Most population based congenital anomaly registries in Europe belong to the EUROCAT network (www.

eurocat-network.eu/), with a common database. In 2009 EUROCAT published a special report on NTD,⁹ which showed that important barriers continue to exist for successful implementation of the recommendations for folic acid supplementation. Hence, only a small minority of women take folic acid supplements in the peri-conceptional period as recommended.¹¹¹²

In this study, we assessed the trends in the total prevalence and live birth prevalence of NTD in Europe by using data from EUROCAT registries for the period 1991- 2011. We examined trends for all NTD combined, as well as separately for anencephaly and spina bifida, the most common forms of NTD.

Methods data sources

Since 1980, the EUROCAT central database has held individual anonymised records of cases of congenital anomaly occurring for full member registries and aggre- gate data for associate member registries, including live births, fetal deaths from 20 weeks gestational age, and termination of pregnancy for fetal anomaly. Informa- tion on each of the registries, including methods of case ascertainment and local procedures regarding ethics approval for the registries’ activities and their collabo- rations with EUROCAT, are available on the EUROCAT website (www.eurocat-network.eu/ABOUTUS/Member- Registries/MembersAndRegistryDescriptions/AllMem- bers) and in the publication by Greenlees et al.¹³ All registries use ICD-9 (international classification of dis- eases, 9th revision) or ICD-10 with BPA extension to code up to nine syndrome or malformation codes for each case.

All cases that were not associated with a chromosomal anomaly and had a diagnostic code corresponding to a neural tube defect (ICD-9 740 -742 and ICD-10 Q00, Q01, Q05) were included for full and associate EUROCAT registries that could provide data from both the 1990s and the 2000s; most registries provided data for the entire, or almost the entire, time period between 1991 and 2011 (table 1). We extracted data on 11 353 cases of NTD, including 4162 cases of anencephaly (ICD-9 740 and ICD-10 Q00), 5776 cases of spina bifida (ICD-9 741 and ICD-10 Q05), and 1415 cases of encephalocele (ICD-9 7420 and ICD-10 Q01), from 28 registries in 19 countries covering approximately 12.5 million births, in October 2013 for the analysis of time trends in total and live birth prevalence of NTD between 1991 and 2011.

patient involvement

No patients were involved in setting the research question or the outcome measures; nor were they involved in design and implementation of the study. There are no plans to involve patients in dissemination.

data analysis

We plotted the time trends, during the period 1991 to 2011, in the total and live birth prevalence of all non-chromosomal NTD and separately for anencephaly and spina bifida. We defined total prevalence of NTD as the total number of cases of NTD (live births plus fetal deaths after 20 weeks of gestational age plus termination of pregnancy for fetal anomaly) per 10 000 total births (live births plus fetal deaths). We defined live birth prevalence as the number of live births with NTD per 10 000 live births. We examined the plots of time trends in total and live birth prevalence of NTD by using restricted cubic splines,¹⁴¹⁵ which can provide a flexi- ble, semi-parametric, continuous model of the relation between prevalence of NTD and time.

Using the number of births as the “exposure” vari- able, we then used random effects Poisson regression models to examine the annual trends in the prevalence of NTD, for all NTD combined and separately for anencephaly and spina bifida. We used random effects models to take into account any heterogeneity that may exist across the registries.¹⁶¹⁷ In the Poisson models, we used linear splines (or “piece-wise exponential models”) for modelling the time trends in the prevalence of NTD; we used splines with five equally spaced knots based on the quintiles of the distribution of the time period to estimate separately the annual trends for the following time periods: 1991-95, 1995-99, 1999-2003, 2003-07, and 2007-11. We used Stata software (versions 11 and 13) for all analyses.

Results

trends in total prevalence of ntd

Figure 1 shows the time trend in the total prevalence of non-chromosomal NTD for the period 1991-2011, using pooled data from all of the EUROCAT registries included in the study and with time trends modelled with restricted cubic splines. The total prevalence of NTD was 9.1 (95% confidence interval 8.9 to 9.3) per 10 000 for 1991-2011 (table 1 ), fluctuating between a maximum of approximately 10.1 (highest) per 10 000 in the period 1999-2003 and 8.5 (lowest) per 10 000 in the period 2003-07 (fig 1). Overall, the total prevalence of NTD in 2011 was comparable to that in 1991. Time trend patterns in the pooled prevalence of anencephaly and spina bifida were comparable to those for all NTD combined.

Table 2 shows the results of random effects Poisson regression models that take into account heterogeneities across registries in estimating the time trends. Esti- mates are presented as prevalence ratios and can be interpreted as percentage increase (for ratios >1) or percentage decrease (for ratios <1). The estimates suggest that the total prevalence of NTD increased by about 4%

a year between 1995 and 1999 (prevalence ratio 1.04, 95% confidence interval 1.01 to 1.07), decreased by 3%

in 1999-2003 (0.97, 0.95 to 0.99), and was fairly stable thereafter.

For non-chromosomal anencephaly, a slight increase was apparent in the 1990s, which was offset by a 4%

decrease between 2003 and 2007 (prevalence ratio 0.96,

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table 1 | total and live birth prevalence of neutral tube defects in study population, european Surveillance of Congenital anomalies (euroCat), 1991-2011 placetime periodtotal birthstotal live births neural tube defects*anencephalySpina bifida no prevalence per 10 000 birthsno prevalence per 10 000 birthsno

prevalence per 10 000 births Austria, Styria1991-2009216 196215 3881667.68401.85984.53 Belgium, Antwerp1991-2011337 862336 4812838.381002.961564.62 Belgium, Hainaut 1991-2011263 703262 4932479.37823.111355.12 Czech Republic2000-091 029 2471 026 3527697.472452.384103.98 Croatia, Zagreb1991-2010131 525130 913634.79181.37362.74 Denmark, Odense1991-2011115 846115 23112710.96443.80685.87 Finland1993-20101 070 9401 066 9869288.673142.934364.07 France, Paris1991-2011666 353660 95080112.023475.213314.97 Germany, Mainz†1991-201171 62771 34313418.72283.918511.87 Germany, Saxony-Anhalt1991-2011289 124287 8502588.93672.321515.22 Hungary1998-20101 260 7191 254 1118476.722562.034983.95 Ireland, Cork and Kerry1996-2010131 168130 44315912.13654.96836.33 Ireland, Dublin1991-2011470 231467 9014118.741493.172014.28 Ireland, south east1997-2011101 348101 269979.58302.96636.22 Italy, Emilia Romagna1991-2011656 637654 6273505.331091.662023.08 Italy, Tuscany1991-2011558 669556 5823135.601081.931632.92 Malta1991-201088 57388 2029710.95252.82566.32 Netherlands, northern1991-2011401 404399 0553388.421082.691974.91 Norway1999-2011775 060769 2937189.272823.643604.65 Poland, Wielkopolska1999-2010440 163437 9664079.25711.612906.59 Portugal, southern1991-2010316 853315 4911504.73621.96762.40 Spain, Basque Country1991-2010361 416359 8103569.851895.231413.90 Switzerland, Vaud1991-2011159 273158 61716210.17623.89744.65 United Kingdom, East Midlands and South Yorkshire1998-2011922 288916 786101210.974264.624805.21 UK, northern England2000-2011382 973380 84352213.632145.592516.56 UK, Thames Valley1991-2011291 827290 30733711.551555.311565.35 UK, Wales1998-2011466 358463 94170315.082785.963307.08 UK, Wessex1994-2011492 629490 43659812.142885.852495.06 Total (95% CI)1991-201112 470 01212 409 66711 3539.105 (8.94 to 9.27)41623.34 (3.24 to 3.44)57764.63 (4.51 to 4.75) *Total number comprises cases of anencephaly, spina bifida, and encephalocele. †Higher prevalence of neural tube defects (NTD), specifically spina bifida, has been consistent observation in Mainz; substantial fluctuations in prevalence of NTD in Mainz occurred over time, however (as in other registries), and no evidence was found that lack of clear downward trend in prevalence of NTD in overall data could be explained by any changes in Mainz.

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0.93 to 0.99), with stable rates afterwards until 2011. For non-chromosomal spina bifida, trends in the total prevalence mirrored those for overall NTD noted above (table 2).

trends in live birth prevalence of ntd

Live birth prevalence of NTD was less than half of all cases of NTD and decreased substantially over the study period (fig 2 ). Estimates from the random effects Poisson models suggested that the decrease occurred more during the 1990s (table 2), once heterogeneities across registries were taken into account. Estimates from the random effects Poisson models also suggested that trends in live birth prevalence of spina bifida were similar to those for all NTD, whereas for anencephaly greater decreases were seen in the live birth prevalence, including a large decrease in the most recent period of 2007-11 equivalent to a 13% annual decrease (prevalence ratio 0.87, 0.76 to 1.00), compared with a stable total prevalence of anencephaly during the same period.

discussion

Using data for more than 11 000 cases of non-chromosomal neural tube defects from 28 population based

table 2 | random effects poisson regression models with splines (piece-wise exponential models) of trends in total and live birth prevalence of non-chromosomal neural tube defects, anencephaly, and spina bifida in 28 member registries in euroCat, 1991-2011 time period

total prevalence live birth prevalence annual prevalence

ratio (95% Ci) p value* annual prevalence

ratio (95% Ci) p value*

all neural tube defects

1991-95 0.98 (0.95 to 1.02)

<0.001

0.93 (0.88 to 0.98)

<0.001

1995-99 1.04 (1.01 to 1.07) 0.97 (0.93 to 1.01)

1999-2003 0.97 (0.95 to 0.99) 0.96 (0.93 to 1.00)

2003-07 0.99 (0.97 to 1.01) 0.97 (0.93 to 1.00)

2007-11 0.99 (0.97 to 1.01) 0.98 (0.93 to 1.02)

anencephaly

1991-95 1.01 (0.95 to 1.07)

<0.001

0.93 (0.80 to 1.09)

<0.001

1995-99 1.03 (0.99 to 1.08) 1.00 (0.89 to 1.13)

1999-2003 0.98 (0.95 to 1.02) 0.91 (0.82 to 1.00)

2003-07 0.96 (0.93 to 0.99) 0.99 (0.89 to 1.09)

2007-11 1.01 (0.98 to 1.05) 0.87 (0.76 to 1.00)

Spina bifida

1991-95 0.97 (0.92 to 1.02)

<0.001

0.94 (0.88 to 1.00)

<0.001

1995-99 1.04 (1.00 to 1.07) 0.96 (0.92 to 1.01)

1999-2003 0.97 (0.94 to 1.00) 0.97 (0.93 to 1.01)

2003-07 0.99 (0.96 to 1.01) 0.96 (0.93 to 1.00)

2007-11 0.99 (0.96 to 1.03) 1.00 (0.95 to 1.05)

*P value tests statistical significance of any trend. In this case, significant P values indicate statistically significant non-monotonic trend for total prevalence and downward trend for live birth prevalence of neural tube defects, anencephaly, and spina bifida.

Year

Prevalence per 10 000

Neural tube defects

1991 1995 1999 2003 2007 2011

0 4 8 12 16 20

Anencephaly

0 1 2 3 4 5

2 4 6 8 10

Spina bifida

0

Year

Neural tube defects

1991 1995 1999 2003 2007 2011

0

Anencephaly

0 0.4 0.8 1.2 1.6 2.0

1 2 3 4 5 2 4 6 8 10

Spina bifida

0

Fig 2 | trends in live birth prevalence of non-chromosomal neural tube defects in europe, 1991-2011: restrictive cubic spline estimates of pooled data in euroCat registries Fig 1 | trends in total

prevalence of non- chromosomal neural tube defects in europe, 1991-2011: restrictive cubic spline estimates of pooled data in euroCat registries

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registries in 18 countries covering approximately 12.5 million births in Europe over the period 1991 to 2011, we found that the overall (pooled) total prevalence of NTD in 2011 was fairly similar to that in 1991 (~9 per 10 000 births). This was also true for the two main types of NTD, anencephaly and spina bifida, each of which accounts for approximately half of the cases of NTD.

Whereas estimates from mixed models that took into account heterogeneities across registries suggested that a small decrease (~3% per year) in total prevalence of NTD between 1999 and 2003, this decrease followed a period of a comparable increase between 1995 and 1999.

Overall, we found no clear evidence of a downward trend over the 20 year study period. In contrast, as a result of prenatal diagnosis and termination of pregnancy for fetal anomaly of most NTD in Europe, their live birth prevalence substantially decreased over time, especially for anencephaly.

Strengths and limitations of study

This was a large study looking at long term trends in the prevalence of NTD based on data from a number of population based registries in Europe. We took into account heterogeneities across registries by using random effects models. Nevertheless, we cannot exclude the possibility that registration problems or other data artefacts may be a partial explanation of the observed trends (or lack thereof) in the prevalence of NTD. How- ever, we had no a priori reason to believe that any changes in registrations would have occurred in specific periods of time in a way that would explain the observed, long term time trends in our study.

Comparisons with other studies

Our results are consistent with those of an older study of trends in the prevalence of NTD in Europe.¹⁰ The overall long term fluctuations in the total prevalence of NTD in our data cannot be due to mandatory fortification of food staples, as such a policy has not yet been implemented in European countries.⁹ Voluntary fortification or the various recommendations issued for folic acid supplementation for women of reproductive age in European countries may have had an effect,⁹¹⁸¹⁹ even if small and as yet very little documented, on the intake of folic acid and thereby the prevalence of NTD.⁹¹²¹⁸ How- ever, the available evidence points to very low uptake of folic acid supplementation in European countries.¹¹¹²

In any case, this was an ecological study and, even if folic acid is known to be an important factor for explaining trends in NTD, it is not the only possible reason for any changes in NTD over time. Changes in other risk factors of NTD (for example, maternal smoking)^20-22 and changes in the incidence and manage- ment of maternal chronic health conditions, such as obesity, diabetes, and epilepsy, or psychiatric illnesses treated with anti-epileptic drugs that are known to be associated with a higher risk of NTD,^23-25 as well as any changes in population characteristics (for example, owing to immigration),² must also be considered as possible explanations for the observed fluctuations in the prevalence of NTD.

Conclusions and policy implications

Our results underscore the fact that 20 years after publication of the Medical Research Council study,⁶ which provided definitive evidence for the efficacy of folic acid in preventing NTD, and years after various recommendations have been issued to promote folic acid supplementation to ensure adequate peri-conceptional folate concentrations for pregnant women, Europe has failed to implement an effective policy for prevention of NTD by folic acid.

NTD represent one of the most prevalent group of birth defects with serious consequences for newborns and their families. Although termination of pregnancy for fetal anomaly has considerably reduced the live birth prevalence of these anomalies, it is certainly not an optimal solution for a birth defect that is highly preventable with a readily available and low cost measure, as is the case for NTD with folic acid supplementation or food fortification. Consumption of adequate natural folates should also be encouraged but in many countries will not raise folate concentrations sufficiently and is likely to leave vulnerable populations unprotected.

Our data suggest that recommendations, voluntary fortification, or both have not been effective in decreasing the prevalence of NTD in Europe. Hence, policies for mandatory fortification of food staples with folic acid should be considered as an important and more effective means for prevention of NTD,⁸²⁶²⁷ while weighing the evidence for its proven benefits and possible risks.² ¹⁹

author aFFiliationS

1Obstetrical, Perinatal and Pediatric Epidemiology Research Team, Center for Biostatistics and Epidemiology, INSERM U1153, Maternité de Port-Royal, 75014 Paris, France

2EUROCAT Central Registry, Centre for Maternal, Fetal and Infant Research, Institute of Nursing Research, University of Ulster, Newtownabbey, UK

3EUROCAT Northern Netherlands Registry, University of Groningen, University Medical Center Groningen, Department of Genetics, Groningen, Netherlands

4Public Health Division of Gipuzkoa, Instituto BIO-Donostia Basque Government CIBER Epidemiología y Salud Pública - CIBERESP, San Sebatian, Spain

5Service de Genetique Medicale Maternite, CHUV, Lausanne, Switzerland

6Children’s University Hospital of Zagreb, Clinical Hospital Sisters of Mercy, Zagreb, Croatia

7National Institute of Health Development, Department of Hungarian Congenital Abnormality Registry and Surveillance, Budapest, Hungary

8CNR Institute of Clinical Physiology and Tuscany Registry of Congenital Defects, “Gabrielle Monasterio” Foundation, Pisa, Italy

9Instituto Nacionale de Saude Dr. Ricardo Jorge, Lisbon, Portugal

10Department of Health Sciences, University of Leicester, Leicester, UK

11Hospital Lillebaelt, Kolding, Denmark

12Department of Health Information and Research, Guardamangia, Malta

13Medical University of Graz, Graz, Austria

14Medical Birth Registry of Norway, Norwegian Institute of Public Health and Department of Global Public Health and Primary Care, University of Bergen, Bergen, Norway

15Department of Medical Genetics, University of Medical Sciences, Poznan, Poland

16Public Health Department, HSE South, Lacken, Kilkenny, Ireland

17Health Service Executive, Dublin, Ireland

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18Provincial Institute for Hygiene, Antwerp, Belgium

19Registro IMER - IMER Registry (Emilia Romagna Registry of Birth Defects), Center for Clinical and Epidemiological Research, University of Ferrara, Ferrara, Italy

20 Department of Public Health, Health Service Executive - South, Ireland

21Birth Registry Mainz Model, Childrens Hospital, University Medical Center, Johannes Gutenberg-University, Mainz, Germany

22Institute of Health and Society, Newcastle University, Newcastle, UK

23Malformation Monitoring Centre Saxony-Anhalt, Medical Faculty Otto-von-Guericke University, Magdeburg, Germany

24National Institute for Health and Welfare, Helsinki, Finland

25National Perinatal Epidemiology Unit, University of Oxford, Oxford, UK

26National Registry of Congenital Anomalies of the Czech Republic, Department of Medical Genetics, Thomayer University Hospital, Prague, Czech Republic

27Public Health Wales, Swanseaa, UK

28Center for Human Genetics, Institut de Recherche Scientifique en Pathologie et en Génétique, Charleroi, Belgium

29University Hospitals Southampton, Faculty of Medicine and Wessex Clinical Genetics Service, Southampton, UK

Contributors: BK, HdW, ML, and HD conceived the study. BK did the statistical analysis, with the assistance of ML, and wrote the first draft of the article. HD, ML, and HdW made substantial contributions to interpretation of results and revision of the manuscript. All other co-authors were registry representatives from EUROCAT participating registries. They contributed and validated their data and participated in the interpretation of results and critical revision of manuscript. BK and ML are the guarantors.

Funding: The study was funded by the European Public Health Commission, EUROCAT Joint Action 2011-2013 (grant No 2010 22 04).

The funding body had no role in study design, analysis, or reporting.

Competing interests: All authors have completed the ICMJE uniform disclosure form at www.icmje.org/coi_disclosure.pdf (available on request from corresponding author) and declare: HD and ML received support from the European Commission DG Sanco during the conduct of this study; no financial relationships with any organisations that might have an interest in the submitted work in the previous three years; no other relationships or activities that could appear to have influenced the submitted work.

Data sharing: No additional data available.

Transparency: The lead authors (the manuscript’s guarantors) affirm that the manuscript is an honest, accurate, and transparent account of the study being reported; that no important aspects of the study have been omitted; and that any discrepancies from the study as planned (and, if relevant, registered) have been explained.

This is an Open Access article distributed in accordance with the Creative Commons Attribution Non Commercial (CC BY-NC 4.0) license, which permits others to distribute, remix, adapt, build upon this work non-commercially, and license their derivative works on different terms, provided the original work is properly cited and the use is non- commercial. See: http://creativecommons.org/licenses/by-nc/4.0/.

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